Rudolph the Red-Nosed Reindeer, of course. Also out of the Rankin-Bass canon: Santa Claus Is Coming To Town, and the little-seen Life & Adventures of Santa Claus.
Also definite seconds for A Charlie Brown Christmas and It's the Great Pumpkin, Charlie Brown ("I got a rock!"), as well as the animated How the Grinch Stole Christmas.
Best Christmas episode of an animated series ever: "A Very Possible Christmas", from Kim Possible. (Shego to Drakken, who's channeling Dr. Seuss: "You've stopped using words." Ron, on the phone: "I'd like the number for 911 immediately, please!" And the whole Snowman Hank shtick, expertly riffing on Rudolph....)